The invention relates to protecting converter means of an electric machine drive from overvoltages and overcurrents.
Converters are used in various generator and motor drives. One such electric machine drive comprising converters is a double-fed slip-ring generator configuration whose rotor circuit comprises two converters having a direct voltage intermediate circuit therebetween. One of these converters is situated electrically between the direct voltage intermediate circuit and a rotor while the other converter is situated electrically between the direct voltage intermediate circuit and an electrical network to be supplied.
Such a double-fed slip-ring generator configuration, which has a particular nominal rotational speed, is usually configured to operate within a particular rotation speed range having an upper limit and a lower limit which depend on the selected transformation ratio of a stator and a rotor. The rotational speed range of a double-fed slip-ring generator configuration whose nominal rotational speed is 1500 1/min may be e.g. 1000 to 2000 1/min. When the generator rotates at a rotational speed lower than the nominal rotational speed, some of the power fed by the stator is fed to the rotor via the converters and slip rings of the generator. Similarly, when the generator rotates at a rotational speed higher than the nominal rotational speed, power is fed via the slip rings of the rotor and the converters from the rotor towards the network supplied by the generator.
When the rotational speed range of the generator configuration corresponds to the above-disclosed values, i.e. the configuration is configured to operate within a rotational speed range which deviates by one third from the nominal rotational speed of the generator, the two converters of a rotor circuit may be dimensioned to correspond with only one third of the power of the generator. This saves investment costs but the relatively low power handling capacity of the converters has to be taken into account when designing the protection of the generator configuration.
During a network failure, the voltage of a direct voltage intermediate circuit may due to the transformation ratio of the generation rise high enough to damage the converters in the rotor circuit. It is well known to protect the converters in a rotor circuit by a protection circuit comprising a thyristor so that in a failure situation the rotor circuit is short-circuited between the rotor and the converters via the thyristor. In connection with a triggering of the thyristor, switches of the converter are opened so that the current of the converters ends. This enables the converters of the rotor circuit to be protected from an overvoltage and zero diodes of the converters to be protected from an overcurrent.
The use of the above-described protection circuit implemented by a thyristor has various problems which depend on the type and properties of the electric machine drive whose converter is being protected by the protection circuit. In all cases, a basic problem is the commutation of a protection thyristor. For instance, in the case of the above-described double-fed slip-ring generator configuration, the problem is that the normal operation of the generator configuration cannot be restarted after a failure unless the generator is first made currentless. The prerequisite of bringing the generator in to a currentless state is based on the fact that otherwise the protection thyristor does not commutate reliably.